Recent reports suggest that chemotherapy and endocrine therapy are directed at different cell populations within human breast cancer. These reports also suggest that the presence or absence of hormone receptor proteins can indicate the degree of biochemical differentiation of the tumor. The purpose of this study is to determine if endocrine therapy or chemotherapy induce detectable, specific, consistent changes in the degree of biochemical differentiation of human breast cancer as judged by the concentration of estrogen and progesterone receptors. A secondary objective is to determine if the presence, absence or concentration of nuclear estrogen receptor or Type II estrogen receptor is correlated with the results of chemotherapy and/or endocrine therapy. This will be accomplished by the systematic, repetitive analysis of the appropriate receptors in human breast cancer biopsy specimens obtained before endocrine therapy or chemotherapy, and after failure of the therapy or recurrence. Receptors will be determined by single-saturating dose assays using either the sucrose gradient or hydroxylapatite assay methods. A direct fluorescent assay, recently developed, will also be used. Changes in the receptor concentrations or the fraction of estrogen receptor positive cells will be correlated with the type of therapy. Response to the various therapies will be correlated with the concentrations of the appropriate receptors.